The present invention relates to a liquid crystal panel, and in particular to a liquid crystal panel with a steerable light source.
The local transmittance of a liquid crystal display (LCD) panel or a liquid crystal on silicon (LCOS) display can be varied to modulate the intensity of light passing from a backlit source through an area of the panel to produce a pixel that can be displayed at a variable intensity. Whether light from the source passes through the panel to an observer or is blocked is determined by the orientations of molecules of liquid crystals in a light valve.
Since liquid crystals do not emit light, a visible display requires an external light source. Small and inexpensive LCD panels often rely on light that is reflected back toward the viewer after passing through the panel. Since the panel is not completely transparent, a substantial part of the light is absorbed during its transits of the panel and images displayed on this type of panel may be difficult to see except under the best lighting conditions. On the other hand, LCD panels used for computer displays and video screens are typically backlit with flourescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel. To provide a display with a more uniform light level, light from these point or line sources is typically dispersed in a diffuser panel before impinging on the light valve that controls transmission to a viewer.
The transmittance of the light valve is controlled by a layer of liquid crystals interposed between a pair of polarizers. Light from the source impinging on the first polarizer comprises electromagnetic waves vibrating in a plurality of planes. Only that portion of the light vibrating in the plane of the optical axis of a polarizer can pass through the polarizer. In an LCD the optical axes of the first and second polarizers are arranged at an angle so that light passing through the first polarizer would normally be blocked from passing through the second polarizer in the series. However, a layer of translucent liquid crystals occupies a cell gap separating the two polarizers. The physical orientation of the molecules of liquid crystal can be controlled and the plane of vibration of light transiting the columns of molecules spanning the layer can be rotated to either align or not align with the optical axes of the polarizers.
The surfaces of the first and second polarizers forming the walls of the cell gap are grooved so that the molecules of liquid crystal immediately adjacent to the cell gap walls will align with the grooves and, thereby, be aligned with the optical axis of the respective polarizer. Molecular forces cause adjacent liquid crystal molecules to attempt to align with their neighbors with the result that the orientation of the molecules in the column spanning the cell gap twist over the length of the column. Likewise, the plane of vibration of light transiting the column of molecules will be “twisted” from the optical axis of the first polarizer to that of the second polarizer. With the liquid crystals in this orientation, light from the source can pass through the series polarizers of the translucent panel assembly to produce a lighted area of the display surface when viewed from the front of the panel.
To darken a pixel and create an image, a voltage, typically controlled by a thin film transistor, is applied to an electrode in an array of electrodes deposited on one wall of the cell gap. The liquid crystal molecules adjacent to the electrode are attracted by the field created by the voltage and rotate to align with the field. As the molecules of liquid crystal are rotated by the electric field, the column of crystals is “untwisted,” and the optical axes of the crystals adjacent the cell wall are rotated out of alignment with the optical axis of the corresponding polarizer progressively reducing the local transmittance of the light valve and the intensity of the corresponding display pixel. Color LCD displays are created by varying the intensity of transmitted light for each of a plurality of primary color elements (typically, red, green, and blue) that make up a display pixel.
Referring to FIG. 1, a liquid crystal display (LCD) 50 (indicated by a bracket) comprises, a backlight 52 and a light valve 54 (indicated by a bracket). Since liquid crystals do not emit light, most LCD panels are backlit with flourescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel. To disperse the light and obtain a more uniform intensity over the surface of the display, light from the backlight 52 typically passes through a diffuser 56 before impinging on the light valve 54.
The transmittance of light from the backlight 52 to the eye of a viewer 58, observing an image displayed on the front of the panel, is controlled by the light valve 54. The light valve 54 comprises a pair of polarizers 60 and 62 separated by a layer of liquid crystals 64 contained in a cell gap between the polarizers. Light from the backlight 52 impinging on the first polarizer 62 comprises electromagnetic waves vibrating in a plurality of planes. Only that portion of the light vibrating in the plane of the optical axis of a polarizer can pass through the polarizer. In an LCD light valve, the optical axes of the first 62 and second 60 polarizers are typically arranged at an angle so that light passing through the first polarizer would normally be blocked from passing through the second polarizer in the series. However, the orientation of the translucent crystals in the layer of liquid crystals 64 can be locally controlled to either “twist” the vibratory plane of the light into alignment with the optical axes of the polarizers, permitting light to pass through the light valve creating a bright picture element or pixel, or out of alignment with the optical axis of one of the polarizers, attenuating the light and creating a darker area of the screen or pixel.
The surfaces of a first glass plate 63 and a second glass plate 61 form the walls of the cell gap and are buffed to produce microscopic grooves to physically align the molecules of liquid crystal 64 immediately adjacent to the walls. Molecular forces cause adjacent liquid crystal molecules to attempt to align with their neighbors with the result that the orientation of the molecules in the column of molecules spanning the cell gap twist over the length of the column. Likewise, the plane of vibration of light transiting the column of molecules will be “twisted” from the optical axis of the first polarizer 62 to a plane determined by the orientation of the liquid crystals at the opposite wall of the cell gap. If the wall of the cell gap is buffed to align adjacent crystals with the optical axis of the second polarizer, light from the backlight 52 can pass through the series of polarizers 60 and 62 to produce a lighted area of the display when viewed from the front of the panel (a “normally white” LCD).
To darken a pixel and create an image, a voltage, typically controlled by a thin film transistor, is applied to an electrode in an array of transparent electrodes deposited on the walls of the cell gap. The liquid crystal molecules adjacent to the electrode are attracted by the field produced by the voltage and rotate to align with the field. As the molecules of liquid crystal are rotated by the electric field, the column of crystals is “untwisted,” and the optical axes of the crystals adjacent to the cell wall are rotated progressively out of alignment with the optical axis of the corresponding polarizer progressively reducing the local transmittance of the light valve 54 and attenuating the luminance of the corresponding pixel. Conversely, the polarizers and buffing of the light valve can be arranged to produce a “normally black” LCD having pixels that are dark (light is blocked) when the electrodes are not energized and light when the electrodes are energized. Color LCD displays are created by varying the intensity of transmitted light for each of a plurality of primary color (typically, red, green, and blue) sub-pixels that make up a displayed pixel. A set of color filters 84, a polarizer 82 arranged in front of the touch screen can significantly reduce the reflection of ambient light, also a cover plate 86 may be placed over the polarizer 82.
The aforementioned example was described with respect to a twisted nematic device. However, this description is only an example and other devices may likewise be used, including, but not limited to, multi-domain vertical alignment (MVA), patterened vertical alignment (PVA), in-plane switching (IPS), and super-twisted nematic (STN) type LCDs.
FIG. 2 illustrates a typical liquid crystal display 100 (i.e., LCD) with a side-lit cold cathode florescent light (i.e., CCFL) backlight lamp 110. The light from the CCFL lamp 110 may be coupled into an optical waveguide 120. The emitted light is primarily confined in the optical waveguide 120 via total internal reflection (TIR) and scattering from the bottom surface 130 of the optical waveguide 120 causes the light that has a scattered angle less than the critical angle to pass through the front of the optical waveguide 120 and distribute generally uniformly across the display. Unfortunately, the light provided by the backlight 110 can not be selectively controlled to different regions of the display.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.